Rechargeable lithium-ion batteries are considered as an effective solution to the increasing need for high energy density electrochemical power sources. Typically, the rechargeable batteries include microporous membranes as battery separators that are interposed between an anode and a cathode in a fluid electrolyte. The battery separator separates the anode and the cathode while allowing transport of ionic charge carriers through the separator. In general, the battery separator material is required to be chemically and electrochemically stable towards the electrolyte and should be mechanically strong to withstand induced high tension during assembly and operation of the battery. In addition, the separator material should be thin and should have relatively small electrical resistivity or large ion conductivity, good electrolyte wettability, and thermal stability.
Various microporous membranes or sheet materials have been used as battery separators. Separators currently used in battery systems are formed of polymeric films which when placed in an electrolyte, are capable of exhibiting a high degree of conductivity. Polyethylene (PE) and polypropylene (PP) are two common precursors used for preparing separators for Li-ion batteries. The PE and PP separators have good tensile strength and are electrochemically stable toward the electrolyte and electrode materials, preventing internal short-circuiting or rapid overcharging of the battery. However, such separators have relatively poor compatibility with liquid electrolytes because of their hydrophobic properties. Moreover, manufacturing costs of battery separators using PE and PP are substantially high. Other polymeric materials have been used for the battery separators, but in general, such materials are not capable of forming thin microporous membranes with low electrical resistivity and high tensile strength.